How does a seismograph work? What is the Richter scale? | HowStuffWorks How does a seismograph work? What is the Richter scale? Jason Reed/ Getty Images A seismograph is the device that scientists use to measure earthquakes. The goal of a seismograph is to accurately record the motion of the ground during a quake. If you live in a city, you may have noticed that buildings sometimes shake when a big truck or a subway train rolls by. Good seismographs are therefore isolated and connected to bedrock to prevent this sort of "data pollution." The main problem that must be solved in creating a seismograph is that when the ground shakes, so does the instrument. Therefore, most seismographs involve a large mass of some sort. You could make a very simple seismograph by hanging a large weight from a rope over a table. By attaching a pen to the weight and taping a piece of paper to the table so that the pen can draw on the paper, you could record tremors in the Earth's crust (earthquakes). If you used a roll of paper and a motor that slowly pulled the paper across the table, you would be able to record tremors over time.

However, it would take a pretty large tremor for you to see anything. In a real seismograph, levers or electronics are used to magnify the signal so that very small tremors are detectable. A big mechanical seismograph may have a weight attached that weighs 1,000 pounds (450 kg) or more, and it drives a set of levers that significantly magnify the pen's motion. Up Next How Volcanoes Work The Richter scale is a standard scale used to compare earthquakes. It is a logarithmic scale, meaning that the numbers on the scale measure factors of 10. So, for example, an earthquake that measures 4.0 on the Richter scale is 10 times larger than one that measures 3.0. On the Richter scale, anything below 2.0 is undetectable to a normal person and is called a microquake. Microquakes occur constantly. Moderate earthquakes measure less than 6.0 or so on the Richter scale. Earthquakes measuring more than 6.0 can cause significant damage. The biggest quake in the world since 1900 scored a 9.5 on the Richter scale.

It rocked Chile on May 22, 1960. 1 What does a seismometer measure? | Reference.com What does a seismometer measure? A: Quick Answer A seismometer measures movements in the ground during an earthquake and creates electric signals that can be read to determine an earthquake's magnitude. Seismometers are contained within a device called a seismograph that is attached to the ground. Full Answer When the Earth's surface shifts during an earthquake, a suspended magnet within the seismometer responds to the movements of the ground while coils of wire in the surrounding case remain still. The coils then generate electric signals based on movements of the magnet relative to the case. These signals can be sent to a computer or written on paper to create a seismogram. Seismographs are very accurate and can pick up seismic waves that a human is unable to feel on his own. However, they can only detect movement in one direction, so many seismograph stations have several seismographs set up to record north-south, east-west and vertical motions.

Using several stations over many miles and knowing that seismic waves get weaker the farther they travel from the source, it is possible to estimate the distance, magnitude, direction and type of earthquake that has occurred. This is accomplished by measuring the difference in time it takes for different seismic waves to reach a detector. What is a Seismograph? - Definition, History & Facts - Video & Lesson Transcript | Study.com What is a Seismograph? - Definition, History & Facts Watch short & fun videos Start Your Free Trial Today An error occurred trying to load this video. Try refreshing the page, or contact customer support. You must create an account to continue watching Register for a free trial Are you a student or a teacher? I am a student Start Your Free Trial To Continue Watching As a member, you'll also get unlimited access to over lessons in math, English, science, history, and more. Plus, get practice tests, quizzes, and personalized coaching to help you succeed. Coming up next: What is an Earthquake? - Definition & Explanation You're on a roll. Keep up the good work! Your next lesson will play in 10 seconds 0:00 What's a Seismograph?

1:00 How a Seismograph Works 2:00 Measurement Scales Add to Add to Add to Want to watch this again later? Log in or sign up to add this lesson to a Custom Course. Custom Courses are courses that you create from Study.com lessons. Use them just like other courses to track progress, access quizzes and exams, and share content. Teachers Organize and share selected lessons with your class. Make planning easier by creating your own custom course. Students Create a new course from any lesson page or your dashboard. From any lesson page: Click "Add to" located below the video player and follow the prompts to name your course and save your lesson. From your dashboard: Click on the "Custom Courses" tab, then click "Create course". Next, go to any lesson page and begin adding lessons. Edit your Custom Course directly from your dashboard. Personalize: Name your Custom Course and add an optional description or learning objective. Organize: Create chapters to group lesson within your course. Remove and reorder chapters and lessons at any time. Share your Custom Course or assign lessons and chapters. Teacher Edition: Share or assign lessons and chapters by clicking the "Teacher" tab on the lesson or chapter page you want to assign.

Students' quiz scores and video views will be trackable in your "Teacher" tab. Premium Edition: You can share your Custom Course by copying and pasting the course URL. Only Study.com members will be able to access the entire course. Create an account to start this course today Try it free for 5 days! Lesson Transcript Instructor: Mary Ellen Ellis Read about the seismograph, what it measures, how it works, and how it has helped scientists to better understand earthquakes both in the past and today. ​!!!What's a Seismograph? Earthquakes are fascinating and frightening at the same time. Have you ever wondered how scientists study these earth-shaking events? They use what's called a seismograph, also called a seismometer, which is an instrument that measures and records seismic waves that move through the earth as the result of an earthquake.

A modern seismograph can help scientists detect earthquakes and measure several aspects of the event: The time at which the earthquake occurred The epicenter, which is the location on the surface of the earth below which the earthquake occurred The depth below the earth's surface at which the earthquake occurred The amount of energy released by the earthquake Scientists measure and record this data to learn more about earthquakes, tectonic plates, and Earth's layers. Earthquakes are difficult to predict, but scientists studying them hope to use seismographic measurements to be able to make more accurate predictions. How a Seismograph Works The idea behind a seismograph is fairly simple. A basic seismograph includes a solid base and a heavy weight suspended from a spring over the base. A pen hangs from the weight and a rotating drum with paper sits below it on the base. The tip of the pen touches the drum. When the earth shakes from an earthquake, the drum rotates and the weighted pen moves back and forth due to the motion of seismic waves. The pen records the movement on the drum. The paper recording of an earthquake is called a seismogram. The most high-tech seismographs used by scientists studying earthquakes today are sophisticated and precise.

They are based on the same concept as a basic, simple seismograph, but make use of electronics, magnets, and amplifiers in order to accurately and precisely measure the smallest ripples in the earth caused by earthquakes. Measurement Scales You have probably heard of the Richter scale, a popular unit for measuring the magnitude of an earthquake. It was invented by Charles F. Richter of the California Institute of Technology in 1935 and uses a logarithmic scale to measure seismic wave magnitude. Because the scale is logarithmic, a difference of one unit represents a tenfold difference in the magnitude of a seismic wave. For instance, an earthquake measured as 6.0 on the Richter scale is 10 times more intense than a 5.0 earthquake. A 7.0 earthquake is 100 times more intense than the 5.0 earthquake. The Richter scale is still often cited in news reports of earthquakes, but scientists studying these events use another scale that allows for more accurate measurements. This is called the moment magnitude scale and it can be applied to a wider range of types and sizes of earthquakes.

It is logarithmic, like the Richter scale, and similar to the older scale for earthquakes up to about a magnitude of 8.0. For larger quakes, the moment magnitude scale begins to differ from the Richter scale and provides more accurate measurements. × Waves, Seismometers, and Seismograms Seismic Networks Introduction The fundamental observations used in seismology (the study of earthquakes) are seismograms which are a record of the ground motion at a specific location. Seismograms come in many forms, on "smoked" paper, photographic paper, common ink recordings on standard paper, and in digital format (on computers, tapes, CD ROMs). Careful observation of ground vibrations during the last 80 years or so have lead to our understanding these vibrations, which are caused by seismic waves. We'll discuss waves in more detail in the next section, for now we need a few basics so that we can understand the variety of seismic signals and instruments. Waves A wave is a disturbance that transfers energy through a medium. Waves are very common in nature: light is a wave, sound is a wave, ocean surf is generated by waves, and even matter has wave-like properties.

The "disturbance" can be an alternating electromagnetic field strength (light), a variation in water height (ocean waves), a variation in material density (sound waves), or a distortion of the shape of the ground (seismic waves). If you've felt Earth shake during an earthquake or explosion then you've felt seismic waves. These vibrations travel outward in all directions from their source. Waves generated by large earthquakes can be detected throughout the world and are routinely recorded and analyzed by seismologists. Seismic waves are generated by many different processes: Earthquakes People Vehicles The range of ground motion amplitudes that are of interest in earthquake studies is very large and seismometers we use are very sensitive. They can detect motions that are much smaller than the thickness of a sheet of paper or as tall as a room. We can detect ground motion in Missouri caused by increased surf activity as a hurricane or large storm system approaches the eastern coast of the lower 48 states. Wavelength and Period To understand the difference between these seismic recording instruments, we need to discuss a little about the waves. First, we use the term wavelength the refer to the peak-to-peak distance on a wave measured at a single time - like in a snapshot.

To measure the wavelength directly, we would need a group of instruments that measure the amplitude of the wave at the same time but at different locations. If we record the ground motion at a single location for a range of time, we can measure the time between peaks in the motion, which we call the wave period. Another important term is frequency, which is the inverse of the period, or one divided by the peak-to-peak time between wave crests. Waves are energy transmitting phenomena that have an amplitude and a wavelength. The upper panel shows a snapshot of the wave at a single time. The lower panel can be thought of as the motion of a single point for an interval of time. The period and the wavelength are related by a simple expression connecting the two with the speed of the wave wavelength = speed x period . Note that the units of the quantities on both sides of the equation balance: wavelength is a measure of distance, such as kilometers, speed is usually specified in terms of kilometers/second, and period is measured in seconds. Checking the physical units of equations is an important way to make sure that you've got the mathematics correct.

Multiple-Frequency Signals The example waves sketched in the cartoon above show a monochromatic, or single-frequency wave, but most interesting signals are actually composites of many frequencies. In many ways seismic vibrations are analogous to light and sound waves so I'll discuss these more familiar waves first. Light Light is actually just one of a range of electromagnetic waves that we use each day. Other related waves, which differ primarily from visible light in their wavelength, include microwaves, radar, AM and FM radio, x-rays, etc. Different colors are actually light waves with different frequencies. For example, red light has a longer wavelength than violet light. The wavelength for red light is about 650 nanometers (billionth of a meter), violet is about 400 nanometers, an x-ray is about 1/3 of a nanometer. FM radio and TV waves have wavelengths around 2-12 meters, AM radio signals have a wavelength of about 30 meters. Perhaps you've heard waves with wavelengths longer than red called infrared and those just shorter than violet called ultra-violet. These terms derive from the wavelength of the colors. TV, VCR, Stereo, etc.

remote controls transmit signals in the infrared and high-energy signals like x-rays and ultra-violet radiation are in the part of the spectrum beyond violet. White light is composed of all visible colors and sunlight contains a large range of the visible spectrum. We could use prism to split the light into it's different color components. The same thing happens when water vapor in the atmosphere is aligned with the rays of light arriving from the sun in just the right geometry, the water vapor acts like a prism and splits the sunlight into a rainbow. Observed seismograms like white light, or sunlight are composites of waves of many different frequencies. Sound Another common example of a multiple-frequency signals are ever day sounds. Humans can hear frequencies in the ranges of 20 to 20,000 hertz and the musical note "middle A" is around 440 hertz. We can calculate the wavelength of sound waves that we can hear using the formula wavelength = velocity / frequency. In air sound travels at about 340 meters per second (the precise value depends on temperature, humidity, and pressure). We hear (measured in air) range from about two-thirds of an inch to 50 feet.

Seismic Signals Much like sunlight contains different colors that can be split apart with a prism, seismic waves contain many different "frequencies" that we can record with specially "tuned" seismometers. The idea is completely analogous with light and sound Seismic Red Bass The range of ground motions that are interesting to seismologists is very large because the process of earth deformation occurs at many different rates and scales. The amplitude range of interesting signals in earthquake studies as a function of frequency compared with a similar range of physical dimensions of some common items. Since I am comparing the "spectral" amplitude as a function of frequency with physical dimensions of the common items, the analogy is not perfect, but the range of variation in size is well represented. "D" represents the distance from the earthquake. We usually specify large distances over Earth's surface in units of degrees and 1 degree = 111.19 km. The large range of amplitudes we are interested in exists because we are interested in all the processes occurring in Earth, from small rock fractures that form in mines to the great earthquakes that occur each year. The amount of energy released by these different processes is enormous, and the large range of interesting amplitudes reflects this.

Seismographs Seismographs, which generally consist of two parts, a sensor of ground motion which we call a seismometer, and a seismic recording system. Modern seismometers are sensitive electromechanical devices but the basic idea behind measuring ground movement can be illustrated using a simpler physical system that is actually quite similar to some of the earliest seismograph systems. A simple mechanical system that illustrate the basic ideas behind of seismic recording systems. The physics behind the sensor is Newton's Law of Inertia: "A body in motion tends to stay in motion unless acted upon by a force, and a body at rest tends to remain at rest unless acted upon by another force." The sensor and the recorder do not have to be located next to one another. For example, the recording "drum" in the lobby of the Macelwane Hall records signals from a sensor in the basement. On the 3rd floor of Macelwane Hall, we record signals from sensors distributed throughout the central United States (the center hallway on 3rd floor is open for public viewing 8:30 AM-3:30 PM weekdays). Seismometers are spread throughout the world, but are usually concentrated in regions of intense earthquake activity or research.

These days, the recording system is invariably a computer, custom designed for seismic data collection and harsh weather. Often they are also connected to a satellite communication system. Such systems enable us to receive seismic signals from all over the world, soon after an earthquake. A real-time seismic recording system with digital storage and satellite communications. Ground vibrations are detected by the sensor, digitally recorded, and then transmitted via satellite. Classic Seismograms For most of the last century, seismograms were recorded on sheet of paper, either with ink or photographically. We call such records "analog" records to distinguish them from digital recordings. These records are read just like a book - from top-to-bottom and left-to-right. The classic paper seismogram is read like a book, from left-to-right and top to bottom. A continuous record is constructed by drawing the line as a sheet of paper fastened to a rotating drum constantly moves horizontally on a threaded attachment. When the ground vibrates the pen moves up or down creating th seismic record of the vibrations. Seismograph station and component, date and start time are recorded on the upper left of this paper.

One problem with these mechanical systems was the limited range of ground motion that could be recorded - vibrations smaller than a line thickness and those beyond the physical range of the ink pen were lost. To circumvent these limitations we often operated high and low-gain instruments side-by-side, but that was neither as efficient nor effective as the modern digital electronic instruments. However, modern "digital" or computerized instruments are relatively new, only about 15-20 years old, and most of our data regarding large earthquakes are actually recorded on paper (or film). Additionally, we still use paper recording systems for display purposes so we can see what is going on without a computer. Digital Seismograms Today, most seismic data are recorded digitally, which facilitates quick interpretations of the signals using computers. Digital seismograms are "sampled" at an even time interval that depends on the type of seismic instrument and the interest of the people who deploy the seismometer. The same principle is used to provide "digital" sound on compact disks. The motion of the ground is continuous, but we can pick only certain positions and reconstruct the motion (within certain limits).

A digital seismogram is a record of the ground movement stored as an array of numbers which indicate the time and the movement of the ground for a range of times and are easily analyzed using computers. The principle is the same as that used for digital audio signals that are stored on Music CD's. Also, since with live in a three-dimensional space, to record the complete ground motion, we must record the motion in three directions. Usually, we usually choose: Up-down North-south East-west With three records of ground motion in three directions, a single seismic station that records about 20 samples per second must store or transmit about 3 x 1.7 x 10^6 samples/day ~ 5 x 10^6 samples/day which equals about 5 megabytes per day. For each 200 seismometers of this type around the world, we are recording about 1,000 mbytes/day ~ 1 gigabyte/day Although we can do better using compression algorithms to save storage, keeping up with this flux of data is a challenge. Seismometers Before technological advances in the last few decades, to record seismic signals we developed and deployed many different kinds of seismometers.

In the 1960's, as part of an effort to verify underground nuclear test threshold treaties, two seismometer models became standard for global earthquake analyses, the world-wide network long-period and short-period instruments. These instruments have been replaced by "broad-band" instruments which can detect ground motions over large ranges, or "bands", of periods. However, since much of our historical data are recorded on the older "narrow-band" short-period and long-period instruments, they remain important sources of data. Different seismometers record different frequencies (or periods) of ground motion and are analogous to different colors in a picture . Seismometer Response Curves Seismometers are usually designed to record signals over a specified range of frequencies (or periods) so it is convenient to discuss instruments based on the range of vibration frequencies that they can detect. Thus one way to characterize seismometers is to describe the range of vibration frequencies that they can detect. A plot of the amplification as a versus frequency is called an instrument response. An instrument is sensitive to vibrations at frequencies for which the "response" curve is relatively large. Five sample instrument response curves are shown below.

The frequency of is shown along the horizontal axis, the equivalent period (period = 1/frequency) is shown along the top horizontal axis. The vertical axis shows the ground-motion amplification factor. To characterize an instrument, what's really important is the range of amplitudes, not the specific amplification, which is usually adjusted depending on the location of the seismometer. I have used numbers around one to illustrate the differences between the response curves for different instruments but actual amplification factors are usually much larger than those shown. The broad band instrument senses most frequencies equally well; the long-period and short period instruments are called "narrow" band, because they preferentially sense frequencies near 1/(15 s) and 1 hertz respectively. The yellow region is the low end of the frequency range audible to most humans (we can hear waves around 20 hertz to 20,000 hertz). The left panel is a comparison of a modern broadband seismometer response and the classic World-Wide Standard Seismic Network (WWSSN) long- and short-period instruments.

The same broad-band response is shown in the right panel, to compare the response with a special short-period instrument, the Wood-Anderson, and an accelerometer. The Wood-Anderson short-period instrument was the one that Charles Richter used to develop his magnitude scale for southern California. The accelerometer is an instrument designed to record large amplitude and high-frequency shaking near large earthquakes. Those are the vibrations that are important in building, highway, etc. design. Seismograms The figure below shows the results of different recording instruments on the measurements of ground motion (displacement) for an earthquake that occurred in Texas, in 1995. The observations were recorded on a broad-band instrument and the signals that would have been recorded on the WWSSN instrument types were simulated using a little mathematics since all the vibrations that would be detected by the long- and short-period seismometers are also recorded by the broadband seismometer. The above diagram shows the ground displacement observed near Tucson, Arizona, caused by an earthquake in southwestern Texas.

The top panel shows the vibrations measured using a broad-band seismometer, the middle panel shows the vibrations as they would be detected by the long-period sensor, and the bottom panel the vibrations that would be sensed by a short-period sensor (scaled by a factor of 10 so we can see them better). The displacements are shown in microns, which are 1x10^-6 meters. Accelerometers Another important class of seismometers was developed for recording large amplitude vibrations that are common within a few tens of kilometers of large earthquakes - these are called strong-motion seismometers. Strong-motion instruments were designed to record the high accelerations that are particularly important for designing buildings and other structures. An example set of accelerations from a large earthquake that occurred in near the coast of Mexico in September of 1985 are shown in the diagram below. The above diagram shows the ground displacement recorded at a strong-motion seismometer that was located directly above the part of a fault that ruptured during the 1985 Mw = 8.1, Michaocan, Mexico earthquake. The left panel is a plot of the three components of acceleration (one vertical and two horizontal).

From the curve we can see that strong, high-frequency shaking lasted almost a minute in the region. The peak acceleration was about 150 cm/s^2. Often we will report such numbers as a fraction of the gravitational acceleration at Earth's surface, which we call "g" and which is about 980 cm/s^2. Thus the peak acceleration in this region was about 150/980 g, or about 0.15g, or equivalently 15% of g. One place with which we are all familiar with acceleration changes is an elevator. The acceleration that you experience in an elevator is about 2 m/s^2 or about 0.2g. However, in an elevator the transition from 0g (not moving) to 0.2g is smooth and comfortable. During the earthquake, you can see that the ground accelerations were varying between -0.1g to +0.1g several times each second, for at least 10-15 seconds. That is not very gentle shaking. The middle panel shows the velocity of ground movement, which we can calculate using calculus - the velocity is the integral of the acceleration.

The peak velocity for this site during that earthquake was about 20-25 cm/sec. And if we integrate the velocity, we can compute the displacement, which is shown in the right-most panel. From the displacement plot, we can see that the permanent offsets near the seismometer were up, west, and south, for a total distance of about 125 centimeters (since the ground displacement is a vector and the seismograms show the three components of that vector, we must use the vector length to specify ground offset, which is the square root of (50^2 + 50^2 + 100^2)). Seismometer Networks Seismic recording systems monitor the ground movements continuously and we have records of the motion of the ground at some places dating back nearly 100 years. Many institutions have been monitoring earthquake activity since the development of seismometers and for example, Caltech and UC Berkeley have relatively long histories of monitoring California seismic activity. At Saint Louis University, Father Macelwane and his students operated a seismic station on campus since around 1911 and we have preserved seismograms for many of the large earthquakes that occurred since then.

In 1974, Father Stauder (now one of the University vice presidents) initiated a regional monitoring program to study seismic activity in the central US in general, and the New Madrid region in particular. That work continues and we are about to upgrade the 1970's instruments with modern, broader-band digital equipment. International monitoring of earthquakes also has a long history. I mentioned the WWSSN that was deployed by the US Government in the 1960's to monitor nuclear explosions and help verify nuclear test treaties since the 1960's. Several smaller networks were deployed before then and today's global seismic network is a cooperative, international effort that consists of more than 100 seismic recording stations. Locations of seismic recording stations that are part of the Global Seismic Network - a cooperative federation of international seismology organizations that share data. Click here to go to the Incorporated Institute for Seismology (IRIS) WWW site where the original map and many details about each station can be accessed. The data from many of these seismometers are available for rapid analysis following most large earthquakes. They can be downloaded from the INTERNET and quickly processed on computers.

Click here to visit the US branch of the Global Seismic Network at the IRIS Web Site . From seismograms we can quickly estimate an earthquake origin time (when the rupture began), location (it's depth, latitude, and longitude), and faulting parameters such as as strike, dip, and slip directions. The US Geological Survey routinely locates earthquakes. You can see a list of the most recent events here (click on the event links to see a map) . The seismology group of Harvard has a continuing research contract to "model" all large earthquakes. In addition, for most large events they provide a quick estimate of the faulting parameters - often within a few hours of the earthquake. You can see their catalog of "earthquake solutions" or view a map with focal mechanisms for recent events. Click here to visit to the Harvard Seismology Earth Solutions Project Site . Regional seismometer networks are also operating continuously. Two of the longest operating are UC Berkeley and Caltech . Visit those sites if you have time or need some information on California Earthquakes. A comprehensive list of seismology sites is maintained by Steve Malone at the University of Washington so if you are looking for information from a specific region - try Steve's list .

Seismic Networks in the Mid-Continent Region Closer to home - Saint Louis University and the University of Memphis maintain sites with information on central US (and global earthquakes). Temporary Seismometer Networks After an earthquake we often descend on the epicentral region with portable seismic instruments to carefully and closely monitor aftershock sequences that follow most large earthquakes. Portable seismic recording systems have been designed for this purpose and they are similar to the permanent stations but often run on battery and solar power. The data are retrieved by a scientist who visits the site and downloads the digital signals from a computer hard disk that's part of the portable seismic recording system. In addition to their value in monitoring and studying earthquakes and aftershocks, seismograms can be used to probe the internal structure of Earth. To facilitate such studies we often deploy temporary seismometer networks for several months or years in geographic regions that we want to study more closely. Some recent study areas are Tibet, east Africa, Fiji, Antarctica, and the western United States. You can see a list of current projects at the IRIS web site ( click here ). Exactly how we use seismograms to study earthquakes and Earth's structure is wrapped up with the nature of seismic wave propagation - which we'll discuss next.

How Seismographs Work How Seismographs Work March 11, 2010 07:32am ET MORE Ground view of collapsed building and burned area at Beach and Divisadero Streets in the Marina District of San Francisco after the 1989 Loma Prieta earthquake. Credit: C.E. Meyer, U.S. Geological Survey Scientists who weren't in Chile during this morning's aftershocks nevertheless knew the moment the rumbling started, thanks to a global network of quake-detecting instruments called seismographs. Seismographs are securely mounted to the surface of the Earth, so when the ground starts shaking , the instrument's case moves. What doesn't move, however, is a suspended mass inside the seismograph, called the seismometer. During an earthquake, the seismometer remains still while the case around it moves with the ground shaking. Traditionally, the suspended mass was a pendulum, but most modern seismometers work electromagnetically. A large permanent magnet is used for the mass and the outside case contains numerous coils of fine wire.

Movements of the magnet relative to the case generate small electric signals in the wire, which can be sent to a computer or recorded onto paper to create a seismogram. Seismographs can detect quakes that are too small for humans to feel. During an earthquake, ground-shaking seismic waves radiate outward from the quake source, called the epicenter. Different types of seismic waves travel at different speeds and through different parts of the Earth during a quake . The farther seismic waves get from their source, the weaker they become. A network of interconnected seismographs can quickly pinpoint the source of a quake by measuring differences in the time it takes different seismic waves to reach a detector. Because seismographs can only measure movement in one direction, many seismograph stations have multiple seismographs to record the north-south, east-west, and vertical motions of the ground. These measurements allow scientists to estimate the distance, direction, magnitude, and the type of earthquake that just occurred. How Are Earthquake Magnitudes Measured? How Are Earthquake Magnitudes Measured? The Richter Scale Figure 1 - Charles Richter studying a seismogram. There are a number of ways to measure the magnitude of an earthquake.

The first widely-used method, the Richter scale, was developed by Charles F. Richter in 1934. It used a formula based on amplitude of the largest wave recorded on a specific type of seismometer and the distance between the earthquake and the seismometer. That scale was specific to California earthquakes; other scales, based on wave amplitudes and total earthquake duration, were developed for use in other situations and they were designed to be consistent with Richter’s scale. The Moment Magnitude Scale Unfortunately, many scales, such as the Richter scale, do not provide accurate estimates for large magnitude earthquakes. Today the moment magnitude scale, abbreviated MW, is preferred because it works over a wider range of earthquake sizes and is applicable globally. The moment magnitude scale is based on the total moment release of the earthquake. Moment is a product of the distance a fault moved and the force required to move it. It is derived from modeling recordings of the earthquake at multiple stations. Moment magnitude estimates are about the same as Richter magnitudes for small to large earthquakes. But only the moment magnitude scale is capable of measuring M8 (read ‘magnitude 8’) and greater events accurately.

Magnitudes are based on a logarithmic scale (base 10). What this means is that for each whole number you go up on the magnitude scale, the amplitude of the ground motion recorded by a seismograph goes up ten times. Using this scale, a magnitude 5 earthquake would result in ten times the level of ground shaking as a magnitude 4 earthquake (and 32 times as much energy would be released). To give you an idea how these numbers can add up, think of it in terms of the energy released by explosives: a magnitude 1 seismic wave releases as much energy as blowing up 6 ounces of TNT. A magnitude 8 earthquake releases as much energy as detonating 6 million tons of TNT. Pretty impressive, huh? Fortunately, most of the earthquakes that occur each year are magnitude 2.5 or less, too small to be felt by most people. Magnitude scales can be used to desribe earthquakes so small that they are expressed in negative numbers. The scale also has no upper limit, so it can describe earthquakes of unimaginable and (so far) unexperienced intensity, such as magnitude 10.0 and beyond.

Here's a table describing the magnitudes of earthquakes, their effects, and the estimated number of those earthquakes that occur each year. The Mercalli Scale Figure 2 - Giuseppe Mercalli Another way to measure the strength of an earthquake is to use the Mercalli scale. Invented by Giuseppe Mercalli in 1902, this scale uses the observations of the people who experienced the earthquake to estimate its intensity. The Mercalli scale isn't considered as scientific as the Richter scale, though. Some witnesses of the earthquake might exaggerate just how bad things were during the earthquake and you may not find two witnesses who agree on what happened; everybody will say something different. The amount of damage caused by the earthquake may not accurately record how strong it was either. Some things that affect the amount of damage that occurs are: the building designs, the distance from the epicenter, and the type of surface material (rock or dirt) the buildings rest on. Different building designs hold up differently in an earthquake and the further you are from the earthquake, the less damage you'll usually see. Whether a building is built on solid rock or sand makes a big difference in how much damage it takes.

Solid rock usually shakes less than sand, so a building built on top of solid rock shouldn't be as damaged as it might if it was sitting on a sandy lot. Measuring the Size of an Earthquake Jump to Navigation Measuring the Size of an Earthquake Seismic waves are the vibrations from earthquakes that travel through the Earth; they are recorded on instruments called seismographs. Seismographs record a zig-zag trace that shows the varying amplitude of ground oscillations beneath the instrument. Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. The time, locations, and magnitude of an earthquake can be determined from the data recorded by seismograph stations. The Richter Scale The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included for the variation in the distance between the various seismographs and the epicenter of the earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions.

For example, a magnitude 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value. At first, the Richter Scale could be applied only to the records from instruments of identical manufacture. Now, instruments are carefully calibrated with respect to each other. Thus, magnitude can be computed from the record of any calibrated seismograph. Earthquakes with magnitude of about 2.0 or less are usually called microearthquakes; they are not commonly felt by people and are generally recorded only on local seismographs. Events with magnitudes of about 4.5 or greater - there are several thousand such shocks annually - are strong enough to be recorded by sensitive seismographs all over the world. Great earthquakes, such as the 1964 Good Friday earthquake in Alaska, have magnitudes of 8.0 or higher.

On the average, one earthquake of such size occurs somewhere in the world each year. The Richter Scale is not commonly used anymore, as it has been replaced by another scale called the moment magnitude scale which is a more accurate measure of the earthquake size. Magnitude Modern seismographic systems precisely amplify and record ground motion (typically at periods of between 0.1 and 100 seconds) as a function of time. This amplification and recording as a function of time is the source of instrumental amplitude and arrival-time data on near and distant earthquakes. Although similar seismographs have existed since the 1890's, it was only in the 1930's that Charles F. Richter, a California seismologist, introduced the concept of earthquake magnitude. His original definition held only for California earthquakes occurring within 600 km of a particular type of seismograph (the Woods-Anderson torsion instrument). His basic idea was quite simple: by knowing the distance from a seismograph to an earthquake and observing the maximum signal amplitude recorded on the seismograph, an empirical quantitative ranking of the earthquake's inherent size or strength could be made.

Most California earthquakes occur within the top 16 km of the crust; to a first approximation, corrections for variations in earthquake focal depth were, therefore, unnecessary. Richter's original magnitude scale (ML) was then extended to observations of earthquakes of any distance and of focal depths ranging between 0 and 700 km. Because earthquakes excite both body waves, which travel into and through the Earth, and surface waves, which are constrained to follow the natural wave guide of the Earth's uppermost layers, two magnitude scales evolved - the mb and MS scales. The standard body-wave magnitude formula is mb = log10(A/T) + Q(D,h) , where A is the amplitude of ground motion (in microns); T is the corresponding period (in seconds); and Q(D,h) is a correction factor that is a function of distance, D (degrees), between epicenter and station and focal depth, h (in kilometers), of the earthquake. The standard surface-wave formula is MS = log10 (A/T) + 1.66 log10 (D) + 3.30 .

There are many variations of these formulas that take into account effects of specific geographic regions, so that the final computed magnitude is reasonably consistent with Richter's original definition of ML. Negative magnitude values are permissible. A rough idea of frequency of occurrence of large earthquakes is given by the following table: MS 6.0 - 6.4 210 This table is based on data for a recent 47 year period. Perhaps the rates of earthquake occurrence are highly variable and some other 47 year period could give quite different results. The original mb scale utilized compressional body P-wave amplitudes with periods of 4-5 s, but recent observations are generally of 1 s-period P waves. The MS scale has consistently used Rayleigh surface waves in the period range from 18 to 22 s. When initially developed, these magnitude scales were considered to be equivalent; in other words, earthquakes of all sizes were thought to radiate fixed proportions of energy at different periods. But it turns out that larger earthquakes, which have larger rupture surfaces, systematically radiate more long-period energy.

Thus, for very large earthquakes, body-wave magnitudes badly underestimate true earthquake size; the maximum body-wave magnitudes are about 6.5 - 6.8. In fact, the surface-wave magnitudes underestimate the size of very large earthquakes; the maximum observed values are about 8.3 - 8.7. Some investigators have suggested that the 100 s mantle Love waves (a type of surface wave) should be used to estimate magnitude of great earthquakes. However, even this approach ignores the fact that damage to structure is often caused by energy at shorter periods. Thus, modern seismologists are increasingly turning to two separate parameters to describe the physical effects of an earthquake: seismic moment and radiated energy. Fault Geometry and Seismic Moment, MO The orientation of the fault, direction of fault movement, and size of an earthquake can be described by the fault geometry and seismic moment. These parameters are determined from waveform analysis of the seismograms produced by an earthquake. The differing shapes and directions of motion of the waveforms recorded at different distances and azimuths from the earthquake are used to determine the fault geometry, and the wave amplitudes are used to compute moment. The seismic moment is related to fundamental parameters of the faulting process.

MO = µSd , where µ is the shear strength of the faulted rock, S is the area of the fault, and <d> is the average displacement on the fault. Because fault geometry and observer azimuth are a part of the computation, moment is a more consistent measure of earthquake size than is magnitude, and more importantly, moment does not have an intrinsic upper bound. These factors have led to the definition of a new magnitude scale MW, based on seismic moment, where MW = 2/3 log10(MO) - 10.7 . The two largest reported moments are 2.5 X 1030 dyn·cm (dyne·centimeters) for the 1960 Chile earthquake (MS 8.5; MW 9.6) and 7.5 X 1029 dyn·cm for the 1964 Alaska earthquake (MS 8.3; MW 9.2). MS approaches its maximum value at a moment between 1028 and 1029 dyn·cm. Energy, E The amount of energy radiated by an earthquake is a measure of the potential for damage to man-made structures.

Theoretically, its computation requires summing the energy flux over a broad suite of frequencies generated by an earthquake as it ruptures a fault. Because of instrumental limitations, most estimates of energy have historically relied on the empirical relationship developed by Beno Gutenberg and Charles Richter: log10E = 11.8 + 1.5MS where energy, E, is expressed in ergs. The drawback of this method is that MS is computed from an bandwidth between approximately 18 to 22 s. It is now known that the energy radiated by an earthquake is concentrated over a different bandwidth and at higher frequencies. With the worldwide deployment of modern digitally recording seismograph with broad bandwidth response, computerized methods are now able to make accurate and explicit estimates of energy on a routine basis for all major earthquakes. A magnitude based on energy radiated by an earthquake, Me, can now be defined, Me = 2/3 log10E - 2.9. For every increase in magnitude by 1 unit, the associated seismic energy increases by about 32 times. Although Mw and Me are both magnitudes, they describe different physical properites of the earthquake.

Mw, computed from low-frequency seismic data, is a measure of the area ruptured by an earthquake. Me, computed from high frequency seismic data, is a measure of seismic potential for damage. Consequently, Mw and Me often do not have the same numerical value. Intensity The increase in the degree of surface shaking (intensity) for each unit increase of magnitude of a shallow crustal earthquake is unknown. Intensity is based on an earthquake's local accelerations and how long these persist. Intensity and magnitude thus both depend on many variables that include exactly how rock breaks and how energy travels from an earthquake to a receiver. These factors make it difficult for engineers and others who use earthquake intensity and magnitude data to evaluate the error bounds that may exist for their particular applications. An example of how local soil conditions can greatly influence local intensity is given by catastrophic damage in Mexico City from the 1985, MS 8.1 Mexico earthquake centered some 300 km away. Resonances of the soil-filled basin under parts of Mexico City amplified ground motions for periods of 2 seconds by a factor of 75 times.

This shaking led to selective damage to buildings 15 - 25 stories high (same resonant period), resulting in losses to buildings of about $4.0 billion and at least 8,000 fatalities. The occurrence of an earthquake is a complex physical process. When an earthquake occurs, much of the available local stress is used to power the earthquake fracture growth to produce heat rather than to generate seismic waves. Of an earthquake system's total energy, perhaps 10 percent to less that 1 percent is ultimately radiated as seismic energy. So the degree to which an earthquake lowers the Earth's available potential energy is only fractionally observed as radiated seismic energy. Determining the Depth of an Earthquake Earthquakes can occur anywhere between the Earth's surface and about 700 kilometers below the surface. For scientific purposes, this earthquake depth range of 0 - 700 km is divided into three zones: shallow, intermediate, and deep. Shallow earthquakes are between 0 and 70 km deep; intermediate earthquakes, 70 - 300 km deep; and deep earthquakes, 300 - 700 km deep.

In general, the term "deep-focus earthquakes" is applied to earthquakes deeper than 70 km. All earthquakes deeper than 70 km are localized within great slabs of shallow lithosphere that are sinking into the Earth's mantle. The evidence for deep-focus earthquakes was discovered in 1922 by H.H. Turner of Oxford, England. Previously, all earthquakes were considered to have shallow focal depths. The existence of deep-focus earthquakes was confirmed in 1931 from studies of the seismograms of several earthquakes, which in turn led to the construction of travel-time curves for intermediate and deep earthquakes. The most obvious indication on a seismogram that a large earthquake has a deep focus is the small amplitude, or height, of the recorded surface waves and the uncomplicated character of the P and S waves. Although the surface-wave pattern does generally indicate that an earthquake is either shallow or may have some depth, the most accurate method of determining the focal depth of an earthquake is to read a depth phase recorded on the seismogram. The most characteristic depth phase is pP. This is the P wave that is reflected from the surface of the Earth at a point relatively near the epicenter.

At distant seismograph stations, the pP follows the P wave by a time interval that changes slowly with distance but rapidly with depth. This time interval, pP-P (pP minus P), is used to compute depth-of-focus tables. Using the time difference of pP-P as read from the seismogram and the distance between the epicenter and the seismograph station, the depth of the earthquake can be determined from published travel-time curves or depth tables. Another seismic wave used to determine focal depth is the sP phase - an S wave reflected as a P wave from the Earth's surface at a point near the epicenter. This wave is recorded after the pP by about one-half of the pP-P time interval. The depth of an earthquake can be determined from the sP phase in the same manner as the pP phase by using the appropriate travel-time curves or depth tables for sP. If the pP and sP waves can be identified on the seismogram, an accurate focal depth can be determined. by William Spence, Stuart A. Sipkin, and George L.

Choy Earthquakes and Volcanoes Earthquake | Seismology, magnitude and other units of measurement Earthquakes: seismology & measurement Seismology As we have seen, most earthquakes are essentially the product of tectonic stresses which are generated at the boundaries of the Earth's tectonic plates . The released energy travels as shock waves, called seismic waves , which may be felt and measured. The study of seismic waves is known as seismology, a word derived from a Greek word meaning "to shake." Seismographs are the instruments which record earthquakes. Scientists use these instruments as their principal tool to study seismic waves. They are very sensitive instruments that can detect, measure and record ground vibrations and their intensities during an earthquake. A seismograph is a simple pendulum. As the ground shakes the base and frame move with the vibrations but inertia keeps the pendulum bob in place. It moves relative to vibrating ground. The pendulum displacement records the changes over time the tracing out the record of the event is called a seismogram. Image: A seismograph The image above shows the seismograph in Sunset Crater Volcano National Monument's visitor centre.

A pen at the top of the device records a zig-zag line on the moving, paper-covered cylinder whenever an earthquake is detected. The seismogram (paper record) is removed each day and replaced. The earthquake data from the seismogram is then analyzed by scientists. Seismographs can help us determine the time, epicentre , focus , and the type of faulting which produced an earthquake as well as estimate how much energy was released. Magnitude and other units of measurement The severity of an earthquake can vary from events which are barely detectable even using the most sophisticated devices, to devastating events which can level cities and trigger Tsunamis and sometimes even volcanic activity. The severity of an earthquake is called its magnitude. Various scales were proposed to measure the magnitude of earthquakes until 1935, when the Richter Scale was developed by a seismologist named Conrad Richter to measure the intensity of the seismic waves. The amplitude (height) of the largest recorded wave of an earthquake at a specific distance is called the Richter magnitude.

Under the Richter scale, each order of magnitude is 10 times more intensive than the last one, which means that a two is 10 times more intense than a one and a three is one hundred times greater. But it is to point out that, while it is correct to say that for each increase in 1 in the Richter magnitude there is a tenfold increase in amplitude of the wave, it is incorrect to say that each increase of 1 in Richter magnitude represents a tenfold increase in the size of the earthquake (as is commonly incorrectly stated by the press). A better measure of the size of an earthquake is the amount of energy released by the earthquake, which is related to the Richter Scale by the following equation: log E = 11.8 + 1.5 M where Log refers to the logarithm to the base 10, E is the energy released in ergs and M the Richter magnitude. Image: Earthquake Severity - Richter Scale Tectonic earthquakes can range in size from magnitudes less than zero, resulting from fault slippage of a few centimetres, to the largest events (magnitude greater than 9), where fault displacements are on the order of many metres.

Earthquake size is determined not only by amount of displacement but also area of the ruptured fault plane. Hence the larger the rupture area, the larger is the earthquake. A magnitude 7 earthquake ruptures a fault area of about 1000 km2 or about 50 km long and 20 km wide. Also depth is an important factor influencing earthquake severity. We know that earthquakes can originate at various depths within the Earth’s solid core. The deeper the earthquake, the more powerful it is, but it is also far less likely to reach the surface. That’s why shallow earthquakes are more common and more dangerous, because the shallower an earthquake, the more damage to surface structures it can cause. There is no limit to the possible magnitude of an earthquake but historically just over magnitude 9 is the record. The earthquake of most recent history to reach 9 on the Richter scale was the Japan quake of March 2011. This was also the largest recorded Japanese earthquake of all time. Image: Earthquake in Japan, March 2011

Question: What is a seismometer used to measure?
Answer: Earthquakes